The present invention lies in the field of methods and apparatus for preparing large arrays of polymers, receptors, and other compounds. More particularly, it lies in the fields of automated methods for preparing diverse arrays of polymers and of techniques for directing specified materials to predefined locations on a substrate.
Very diverse collections of compounds are often desired in research and other applications. Microtiter plates conventionally contain wells for testing as many as 96 different compounds. For many applications, 96 represents an unacceptably small number of samples. Further, when the compounds of interest are rare or valuable, the test samples must be minuscule. Unfortunately, signals from such small samples can be lost or diluted in the relatively large volume wells of a conventional microtiter plate. If the wells were made smaller and placed in higher densities on microtiter plates, suitable methods would still be needed for accurately delivering small aliquots to specified wells, and for identifying wells containing compounds that exhibit a desired activity.
Often the compounds of interest are polymers, such as nucleic acids, polysaccharides, or peptides. Some attempts have been made to synthesize a limited number of peptide sequences on, for example, a number of "rods." See, for example, Geysen, et al., J. Immun. Meth. (1987) 102:259-274, incorporated herein by reference for all purposes, which describes a procedure in which peptide syntheses are carried out in parallel on several rods or pins (to complement standard microtiter plates, 96 were used in the described method). The Geysen et al. method is limited in the number of sequences that can be synthesized in a reasonable amount of time. For example, Geysen et al. report in the above journal that it has taken approximately 3 years to synthesize 200,000 peptide sequences. In addition, such methods have continued to produce fewer peptide sequences for study than are often desired. Even if the large number of desired compounds could be produced quickly, they would not be readily accessible for further study. The 96 pin arrays of Geysen et al. would occupy far too much space to rapidly screen thousands of candidate polymers.
Techniques have recently been introduced for synthesizing large arrays of different peptides and other polymers on solid surfaces. For example, in Pirrung et al., PCT Publication No. WO 90/15070, incorporated herein by reference for all purposes, a technique is disclosed for generating arrays of peptides and other materials using, for example, light-directed, spatially-addressable synthesis techniques. See also, Fodor et al., PCT Publication No. WO 92/10092 (incorporated herein by reference for all purposes) which discloses, among other things, a method of gathering fluorescence intensity data, various photosensitive protecting groups, masking techniques, and automated techniques for performing light-directed, spatially-addressable synthesis techniques. Arrays containing up to 64,000 different elements have been formed using this technology. See, U.S. patent application Ser. No. 805,727, filed Dec. 6, 1991, which is incorporated herein by reference for all purposes. Because of their relationship to semiconductor fabrication techniques, these methods have come to be referred to as "Very Large Scale Immobilized Polymer Synthesis," or "VLSIPS.TM." technology. Such techniques have met with substantial success in, for example, screening various ligands, such as peptides, to determine their relative binding affinity to a receptor such as an antibody.
In some applications, it is desirable to study pre-formed collections of synthetic chemical compounds or natural product extracts. For example, it would be desirable to "immortalize" a collection of chemical samples from a rain forest threatened with destruction. In addition, thousands of different synthetic compounds often are cataloged in "libraries" of Universities and corporations. Unfortunately, the compounds of these libraries are not readily accessible for systematic study.
Methods for immobilizing collections of materials on a solid substrate are known. For example, U.S. Pat. No. 4,562,157 issued to Lowe et al., and incorporated herein by reference for all purposes, discusses a technique for attaching biochemical ligands to surfaces through a photochemically reactive arylazide. Irradiation of the azide creates a reactive nitrene moiety which reacts irreversibly with macromolecules in solution to form a covalent bond. The high reactivity of the nitrene intermediate, however, results in both low coupling efficiencies and many potentially unwanted reactions through nonspecific reactions.
An improved method for immobilizing collections of compounds is disclosed in Barrett et al., PCT Publication No. WO 91/07087 which is incorporated herein by reference for all purposes. This publication discloses a technique for immobilizing arrays of anti-ligands, such as antibodies or antigens, hormones or hormone receptors, oligonucleotides, polysaccharides, and other materials. Cycles of irradiation on different regions of a surface and immobilization of different anti-ligands allows formation of an immobilized matrix of anti-ligands at defined sites on the surface. The immobilized matrix of anti-ligands permits simultaneous screenings of a liquid sample for ligands having high affinities for certain anti-ligands of the matrix.
While the technique disclosed in PCT Publication No. WO 91/07087 as well as the VLSIPS.TM. technique are useful for preparing and using large arrays of materials, other techniques emphasizing efficient formation and use of much larger arrays of individual compounds would be desirable. If a very large group of compounds is used to form a diverse array, the number of individual locations on the array can be enormous. For example, to synthesize all dimer from 100 monomer starting materials requires 100,000 (100.sup.2) separated locations. Forming an array of this size can be a daunting task. Further, once the array is produced, it can become quite difficult to locate specific members of the array for further processing or study.
Some work has been done to automate synthesis of polymer arrays. For instance, in Fodor et al., PCT Publication No. WO 92/10092, a method is described for using a computer-controlled system to direct a VLSIPS.TM. procedure. Further, Southern, PCT Application No. WO 89/10977 describes the use of a conventional pen plotter to deposit three different monomers at twelve distinct locations on a substrate. These monomers were subsequently reacted to form three different polymers, each twelve monomers in length. This reference also discusses the possibility of using an ink-jet printer to deposit monomers on a substrate. Wong et al., European Patent Application No. 260 965 describes a process in which a single polymer species in solution was putatively deposited in a single spot on a substrate by an apparatus resembling an ink-jet printer. However, neither the method described in the Wong et al. reference nor the method described in the Southern application concerns very large arrays of polymers. Further, the methods described in these two references would be unacceptably slow in accessing specific elements of a large array.